Control device for vehicular lockup clutch

ABSTRACT

A control device for a vehicular lockup clutch, including a lockup clutch that directly connects an input rotation member and an output rotation member of a hydraulic transmission device constituting part of a power transmission path between an engine and drive wheels, a linear solenoid valve that controls an engagement hydraulic pressure for engaging and actuating the lockup clutch, and a controller that is configured to perform a flexible start control for slip-engaging the lockup clutch when a vehicle starts moving, and to perform, before starting the flexible start control, a precharge control for establishing a standby state in which a predetermined preparation pressure is indicated to the linear solenoid valve.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2010-072743 filed onMar. 26, 2010 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a control device for a vehicular lockup clutch,and more particularly to improvement of flexible start control when avehicle starts moving.

2. Description of the Related Art

In a vehicle provided with a lockup clutch that can directly connectinput and output members of a hydraulic transmission device constitutingpart of a power transmission path between an engine and drive wheels anda linear solenoid valve serving to control an engagement hydraulicpressure for engaging and actuating the lockup clutch, a control devicefor a vehicular lockup clutch, which conducts flexible start control forslip-engaging the lockup clutch when the vehicle starts moving, isavailable. For example, such as control device for a vehicular lockupclutch is disclosed in Japanese Patent Application Publication No.2005-016563 (JP-A-2005-016563). The control device for a vehicularlockup clutch disclosed in JP-A-2005-016563 starts the flexible startcontrol, for example, when an accelerator is stepped on and maintains acontrol hydraulic pressure of the linear solenoid valve in advance at apredetermined initial engagement pressure in predetermined initialengagement section before the slip engagement of the lockup clutch isstarted in the flexible start control. With the control device describedin JP-A-2005-016563, by performing the flexible start control in theabove-described manner, it is possible to reduce a control error whenslip engagement of the lockup clutch is started and perform stable slipengagement of the lockup clutch when the vehicle starts moving.

In the control device described in JP-A-2005-016563, a control hydraulicpressure of the linear solenoid valve is raised to the initialengagement pressure at the initial stage of the flexible start control,whereby the responsiveness of hydraulic control is ensured in thesubsequent slip engagement of the lockup clutch. However, it cannot besaid that sufficient responsiveness has been ensured. For example, whena case is considered in which the working oil used for the hydrauliccontrol of the lockup clutch has an ultralow temperature or an ultrahightemperature, the responsiveness will apparently be insufficient,although such data are not publicly available. A problem encounteredwhen the responsiveness of the hydraulic control is low is thatdrivability and fuel consumption are degraded by an overshoot of enginerevolution speed.

Further, in a fast-fill mode in which a hydraulic pressure command valuefor the linear solenoid valve is temporarily set high when the flexiblestart control is started, it is also possible to ensure theresponsiveness of the engagement hydraulic pressure by setting thehydraulic pressure command value in the fast-fill mode even higher, butwhere the hydraulic pressure command value is set high in the fast-fillmode, a hydraulic pressure overshoot easily occurs and it is highlyprobable that controllability of hydraulic pressure will degrade. Suchproblem is not publicly disclosed.

SUMMARY OF THE INVENTION

The invention has been created with the foregoing in view and theinvention provides a control device for a vehicular lockup clutch thatcan ensure sufficient responsiveness of the lockup clutch in theflexible start control.

A first aspect of the invention relates to the following control devicefor a vehicular lockup clutch. This control device includes:

a lockup clutch that directly connects an input rotation member and anoutput rotation member of a hydraulic transmission device constitutingpart of a power transmission path between an engine and drive wheels;

a linear solenoid valve that controls an engagement hydraulic pressurefor engaging and actuating the lockup clutch; and

a controller that is configured to perform a flexible start control forslip-engaging the lockup clutch when a vehicle starts moving, and toperform, before starting the flexible start control, a precharge controlfor establishing a standby state in which a predetermined preparationpressure is indicated to the linear solenoid valve.

With the above-described control device for a vehicular lockup clutch,immediately before the flexible start control is started, the outputhydraulic pressure outputted from the linear solenoid valve rises to thepreparation pressure or a hydraulic pressure close thereto and thereforea sufficient increase in response of the lockup clutch in the flexiblestart control over that in the case without the precharge control can beensured. Therefore, for example, the unnecessarily rapid increase in therevolution speed of the engine caused by a delay of response inhydraulic pressure control of the linear solenoid valve in the flexiblestart control can be adequately inhibited and fuel consumption can beimproved. Further, since the unnecessarily rapid increase in therevolution speed of the engine is inhibited when the vehicle startsmoving, the drivability is improved.

It is also preferred that a switching valve be provided that can blockthe supply of the engagement hydraulic pressure to the lockup clutch,regardless of the operation state of the linear solenoid valve, and thesupply of the engagement hydraulic pressure could be blocked by theswitching valve while the precharge control is being performed. As aresult, the disengaged state of the lockup clutch can be reliablymaintained while the precharge control is being performed and thereforethe lockup clutch can be prevented from being actuated to the engagementside before the vehicle starts moving and the unnecessary decrease inthe engine revolution speed caused thereby can be avoided.

Further, it is preferred that the switching valve be switched to thestate in which the engagement hydraulic pressure can be supplied whenthe flexible start control is started. As a result, the engagementhydraulic pressure that has been set high in advance by the prechargecontrol can be supplied to the lockup clutch immediately after theflexible start control has been started. Therefore, before the flexiblestart control is started, the disengaged state of the lockup clutch canbe reliably maintained, and where the flexible start control is started,the lockup clutch can be immediately actuated with good responsivenessto the engagement side.

Further, it is preferred that the precharge control be started whenbraking of the wheels is canceled. In this case, it is also preferredthat the precharge control be started when braking of the wheels iscanceled, the vehicle has been stopped, and the shift lever is in adrive range. It is also preferred that the flexible start control bestarted when the operation amount of the accelerator pedal is equal toor greater than a predetermined value and the vehicle speed is equal toor higher than a predetermined value. As a result, since it can bepredicted that where braking of the wheels is canceled when the vehiclestarts moving, for example, the accelerator pedal will be stepped on andthe flexible start control will be started immediately thereafter, theprecharge control can be started in a suitable period before theflexible start control is started and responsiveness of the lockupclutch in the flexible start control can be ensured.

BRIEF DESCRIPTION OF DRAWINGS

The features, advantages, and technical and industrial significance ofthis invention will be described in the following detailed descriptionof example embodiments of the invention with reference to theaccompanying drawings, in which like numerals denote like elements, andwherein:

FIG. 1 is a drawing outlining the configuration of a power transmissiondevice of a vehicle that can advantageously use the invention;

FIG. 2 is an engagement operation table illustrating the engagementstage of frictional engagement elements, that is, frictional engagementdevices, when a plurality of transmission stages are selectivelyestablished in the automatic transmission shown in FIG. 1;

FIG. 3 is a block diagram illustrating main components of an electriccontrol system provided in the vehicle for controlling the automatictransmission shown in FIG. 1;

FIG. 4 is a hydraulic circuit diagram illustrating an extracted portionof the hydraulic control circuit provided in the power transmissiondevice shown in FIG. 1, this portion relating to the control of theengagement stage of the lockup clutch;

FIG. 5 is a functional block diagram illustrating main control functionsprovided in the electronic control unit shown in FIG. 3;

FIG. 6 is a time chart for explaining an example of precharge controlperformed by the precharge control device shown in FIG. 5, and

FIG. 7 is a flowchart illustrating main features of control operationsof the present embodiment performed by the electronic control unit shownin FIG. 3, that is, control operations performed by flexible startcontrol after the precharge control has been performed.

DETAILED DESCRIPTION OF EMBODIMENTS

The embodiments of the invention will be described below in greaterdetail with reference to the appended drawings.

FIG. 1 is a drawing outlining the configuration of a power transmissiondevice of a vehicle 8 that can advantageously use the invention. Anautomatic transmission 10 shown in FIG. 1 can be advantageously mountedin the left-right direction (transversely) on a front engine-front drive(FF) vehicle. A first transmission unit 14 constituted mainly by a firstplanetary gear device 12 of a single pinion type and a secondtransmission unit 20 of a Ravigneaux type constituted mainly by a secondplanetary gear device 16 of a double pinion type and a third planetarygear device 18 of a single pinion type are provided on a common centeraxis denoted by C in FIG. 1 inside a transmission case 26 as anon-rotating member attached to a vehicle body. The rotation of an inputshaft 22 is transmitted via the first transmission unit 14 and thesecond transmission unit 20 and outputted from a transmission outputmember 24. The input shaft 22 corresponds to an input member of theautomatic transmission 10. In the present embodiment, the input shaftcorresponds to a turbine shaft of a torque converter 30 that isrotationally driven by an engine 28 that is a drive power source forrunning the vehicle. The transmission output member 24 corresponds to anoutput member of the automatic transmission 10 and functions as anoutput gear, that is, a differential drive gear, meshing with adifferential driven gear (large-diameter gear) 36 for transmitting thepower to a differential gear mechanism 34 shown in FIG. 3. As shown inFIG. 1, a torque converter 30 is provided in a power transmission pathbetween the automatic transmission 10 and the engine 28 serving as adrive power source. The output of the engine 28 is transmitted to theautomatic transmission 10 via the torque converter 30 and thentransmitted to a pair (left and right) of drive wheels 40 via theautomatic transmission 10, differential gear device 34, and a pair (leftand right) wheel shafts 38. The vehicle 8 (see FIG. 3) of the presentembodiment is provided with the above-described automatic transmission10, engine 28, torque converter 30, differential gear device 34, wheelshafts 38, drive wheels 40, and wheel brake devices 42 (see FIG. 3).Further, the automatic transmission 10 and the torque converter 30 areconfigured substantially symmetrically with respect to the center axis Cthereof. In the outline drawing in FIG. 1, the section located below thecenter line C is omitted. The drive wheels 40 and driven wheels (notshown in the figure) are the wheels provided at any vehicle 8, and inthe explanation of the present embodiment, they will be referred to aswheels, without distinguishing the drive wheels 40 and the drivenwheels.

The engine 28 is for example an internal combustion engine, such as agasoline engine or a diesel engine, in which drive power is generated bycombustion of fuel injected into cylinders. The torque converter 30 isprovided with a pump vane wheel 30 p connected to a crank shaft of theengine 28 and a turbine vane wheel 30 t connected to the automatictransmission 10 via the turbine shaft and is a hydraulic transmissiondevice that performs power transmission between the pump vane wheel 30 pand the turbine vane wheel 30 t via a fluid. Thus, in the torqueconverter 30 of the present embodiment the pump vane wheel 30 pcorresponds to an input rotation member that is a rotation member on theinput side, and the turbine vane wheel 30 t corresponds to an outputrotation member that is a rotation member on the output side. The torqueconverter 30 is provided between the pump vane wheel 30 p and theturbine vane wheel 30 t with a lockup clutch (direct connection clutch)32 that can be directly connected so as to rotate integrally with thepump vane wheel 30 p and the turbine vane wheel 30 t. The lockup clutch32 is configured so that the engagement state thereof is controlledbetween disengagement, slip engagement (semi-engagement), and completeengagement by a hydraulic control circuit 100 such as shown in FIG. 4described hereinbelow. When the lockup clutch is completely engaged, thepump vane wheel 30 p and the turbine vane wheel 30 t rotate integrally.

The wheel brake device 42 is a well-recognized drum brake or disk brakeand provided for each wheel. The wheels are braked when a driver stepson a foot brake pedal 54 (see FIG. 3). Thus, the wheels are braked by abrake hydraulic pressure generated by stepping on the foot brake pedal54.

The automatic transmission 10 is for example a multistage automatictransmission in which any of a plurality of preset transmission stages(speed ratio) is selectively established. The transmission is configuredby providing a plurality of hydraulic frictional engagement devices asengagement elements for changing the speed. Thus, as shown in FIG. 1,clutches C1, C2 and brakes B1 to B3 (when these components are notspecifically distinguished, they are simply referred to as clutches Cand brakes B) are provided. These clutches C and brakes B are hydraulicfrictional engagement devices that are engaged by a hydraulic actuator,such as multiplate clutches or brakes. The engaged and disengaged statesof the clutches and brakes are switched and excess hydraulic pressureduring engagement and disengagement thereof is controlled by excitation,de-excitation, and current control of a linear solenoid valve serving asan electromagnetic valve device located inside the below-describedhydraulic control circuit 100. Further, in the automatic transmission10, six forward transmission stages (forward gear stages, gear stagesfor traveling forward), namely, first transmission stage (first speedgear stage) “1st” to sixth transmission stage (sixth speed gear stage)“6th” and one rearward transmission stage (rearward gear stage, gearstage for travelling rearward) “R” are established according to thecombination of connection stages of rotation elements (sun gears S1 toS3, carriers CA1 to CA3, and ring gears R1 to R3) of the firsttransmission unit 14 and the second transmission unit 20 by selectivelyengaging and disengaging the clutches C and brakes B.

FIG. 2 is an engagement operation table illustrating the operation stateof frictional engagement elements, that is, the frictional engagementdevice, when a plurality of transmission stages are selectivelyestablished in the automatic transmission 10. The transmission operationtable represents the relationship between the operation stages of thetransmission stages, clutches C1, C2, and brakes B1 to B3. Symbol “O” inthe table represents the engagement. As shown in FIG. 2, in theautomatic transmission 10, the first speed gear stage “1st” isestablished by engaging the clutch C1 and the brake B2, the second speedgear stage “2nd” is established by engaging the clutch C1 and the brakeB1, the third speed gear stage “3rd” is established by engaging theclutch C1 and the brake B3, the fourth speed gear stage “4th” isestablished by engaging the clutch C1 and the clutch C2, the fifth speedgear stage “5th” is established by engaging the clutch C2 and the brakeB3, and the sixth speed gear stage “6th” is established by engaging theclutch C2 and the brake B1. The rearward gear stage “R” is establishedby engaging the brake B2 and the brake B3, and a neutral state “N” isestablished by disengaging any of the clutches C1, C2 and brakes B1 toB3. The speed ratio of each transmission stage is appropriatelydetermined by gear ratios (=(number of teeth in a sun gear)/(the numberof teeth in a ring gear)) ρ1, ρ2, ρ3 of the first planetary gear device12, the second planetary gear device 16, and the third planetary geardevice 18.

FIG. 3 is a block diagram illustrating main components of the electriccontrol system provided in the vehicle 8 to control the automatictransmission 10 and the like. The electronic control unit 80 shown inFIG. 3 is configured for example by including the so-calledmicrocomputer provided with a central processing unit (CPU), a randomaccess memory (RAM), a read-only memory (ROM), and an input interface,and so that control operations of various kinds, such as output controlof the engine 28 and transmission control of the automatic transmission10, are performed by performing signal processing according to a programthat has been stored in advance in the ROM, while using a temporarystorage function of the RAM with the CPU. The electronic control unit 80includes a function of a control unit that performs engagement anddisengagement of the lockup clutch 32. Further, if necessary, theelectronic control unit 80 is configured as individual control units foreach type of control, that is, a control unit for output control of theengine 28 and a control unit for transmission control of the automatictransmission 10.

As shown in FIG. 3, signals obtained from various sensors and switchesprovided at the vehicle and indicating the detection values thereof aresupplied to the electronic control unit 80. Thus, a signal representingan operation amount (accelerator depression amount) Acc of theaccelerator pedal 50 that is used as the so-called acceleratordepression amount is supplied to the electronic control unit 80 from anaccelerator operation amount sensor 52, a signal indicating whether afoot brake pedal 54 that is a usual brake is supplied from a brakeswitch 56, a signal indicating a revolution speed N_(E) of the engine 28is supplied to an engine revolution speed sensor 58, a signal indicatingan intake air quantity Q of the engine 28 is supplied from an engineintake air amount sensor 60, a signal indicating a temperature T_(A) ofthe intake air of the engine 28 is supplied from an engine intake airtemperature sensor 62, a signal indicating an opening degree (throttlevalve opening degree) θ_(TH) of an electronic throttle valve is suppliedfrom a throttle sensor 64, a signal indicating a vehicle speed Vcorresponding to a revolution speed N_(OUT) of the transmission outputmember 24 is supplied from a vehicle speed sensor 66, a signalindicating a cooling water temperature T_(W) of the engine 28 issupplied from a cooling water temperature sensor 68, a signal indicatinga lever position (operation position) P_(SH) of a shift lever 72 servingas a shift operation member is supplied from a lever position sensor 74,a signal indicating a revolution speed N_(IN) of the input shaft 22,that is, a turbine revolution speed N_(T), is supplied from a turbinerevolution speed sensor 76, and a signal indicating an AT oiltemperature T_(OIL) that is a temperature of working oil inside thehydraulic control circuit 100 is supplied from an AT oil temperaturesensor 78.

The electronic control unit 80 is also configured to output signals forcontrolling the operation of various devices provided at the vehicle.Thus, a signal that drives the throttle actuator for controlling theopening and closing of the electronic throttle valve according to theaccelerator depression amount Acc, an injection signal for controllingthe amount of fuel injected from a fuel injection device, and anignition timing signal for controlling the ignition timing of the engine28 with an ignition device are outputted as engine output controlcommand signals S_(E) for conducting output control of the engine 28.Further, for example, a signal for driving various electromagnetic valvedevices provided in the hydraulic control circuit 100 are outputted astransmission control command signals S_(P) for conducting transmissioncontrol of the automatic transmission 10. Further, signals for driving asolenoid valve SL and a linear solenoid valve SLU (see FIG. 4) providedin the hydraulic control circuit 100 are outputted as lockup clutchengagement control signals S_(L) for conducting engagement control ofthe lockup clutch 32 provided in the torque converter 30. The throttlevalve opening degree θ_(TH) has a one-to-one correspondence relationshipwith the accelerator depression mount Acc, and the electronic controlunit 80 increases the throttle valve opening degree θ_(TH) as theaccelerator depression mount Acc increases, on the basis of theaccelerator depression amount Acc from the preset relationship.

FIG. 4 is a hydraulic circuit diagram illustrating an extracted portionof the hydraulic control circuit 100 provided in the vehicle 8, that is,the control of the engagement state of the lockup clutch 32. As shown inFIG. 4, the hydraulic control circuit 100 is provided with a switchingsolenoid valve SL that is ON/OFF operated by a switching electromagneticsolenoid 102 and generates a switching signal pressure P_(SW), a clutchswitching valve 104 that switches the lockup clutch 32 to adisengagement position (OFF position) corresponding to a disengagedstage or an engagement position (ON position) corresponding to theengaged stage in response to the switching signal pressure P_(SW), alinear solenoid valve SLU for slip control that outputs a signalpressure (output pressure) P_(SLU) corresponding to a drive electriccurrent supplied from the electronic control unit 80, a lockup controlvalve 106 that switches the operation stage of the lockup clutch 32within a range from a slip state to a lockup ON state when the lockupclutch 32 is set to the engaged state by the clutch switching valve 104,and a hydraulic cooler 108 for cooling the working oil.

Further, as shown in FIG. 4, the hydraulic control circuit 100 isprovided with a mechanical oil pump 112 that is driven, for example, bythe engine 28 for sucking in and pumping via a strainer 110 the workingoil that has returned to a hydraulic pan (not shown in the figure), andthe pressure of the working oil under a pressure raised by the oil pump112 is adjusted to a first line pressure PL1 by a first pressureadjusting valve 114 of a relief system. A second pressure adjustingvalve 116 that is similarly a pressure adjusting valve of a reliefsystem is also provided, and a second line pressure P_(L2) is generatedby adjusting the pressure of the working oil flowing out of the firstpressure adjusting valve 114 with the second pressure adjusting valve116. A third pressure adjusting valve 118 is a pressure reducing valvethat takes the first line pressure PL1 as a base pressure, and thisthird pressure adjusting valve 118 generates a modulator pressure P_(M)which is a constant pressure that has been set in advance. A signalpressure from a linear solenoid valve (not shown in the figure) issupplied to the first pressure adjusting valve 114 and the secondpressure adjusting valve 116, and a predetermined signal pressure isoutputted from this linear solenoid valve in response to a command fromthe electronic control unit 80, whereby the first pressure adjustingvalve 114 and the second pressure adjusting valve 116 adjust thehydraulic pressure to a level advantageous for the vehicle travelingconditions on the basis of the accelerator depression amount andrevolution speed of the engine 28.

As shown in FIG. 4, the lockup clutch 32 is a hydraulic friction clutchin which frictional engagement with a front cover 128 is caused by adifference pressure ΔP (=P_(ON)−P_(OFF)) of a hydraulic pressure P_(ON)inside an engagement hydraulic chamber 122 supplied via an engagementoil path 120 and a hydraulic pressure P_(OFF) inside a disengagementhydraulic chamber 126 supplied via a disengagement oil path 124. Theoperation state of the torque converter 30 can be generally classified,for example, into three states: the so-called lockup OFF state in whichthe difference pressure ΔP is negative and the lockup clutch 32 isdisengaged, the so-called slip state in which the difference pressure ΔPis equal to or higher than zero and the lockup clutch 32 issemi-engaged, and the so-called lockup ON state in which the differencepressure ΔP assumes a maximum value and the lockup clutch 32 iscompletely engaged. Further, in the slip state of the lockup clutch 32,by setting the difference pressure ΔP to zero, it is possible toeliminate torque distribution of the lockup clutch 32 and set the torqueconverter 30 to an operation state similar to the lockup OFF state.

The clutch switching valve 104 switches the lockup clutch 32 betweenengagement and disengagement, and a spool valve element 130 is providedinside the clutch switching valve 104 for switching the connectionstate. FIG. 4 shows a state in which the spool valve element 130 is inthe OFF position (OFF) in which the section on the left side from thecenter line is in the disengaged state of the lockup clutch 32 and astate in which the spool valve element 130 is in the ON position (ON) inwhich the section on the right side from the center line is in theengaged state. The clutch switching valve 104 is provided with adisengagement port 132 communicating with the disengagement hydraulicchamber 126, an engagement port 134 communicating with the engagementhydraulic chamber 122, an input port 136 to which the second linepressure P_(L2) is supplied, a discharge port 138 that discharges theworking oil located inside the engagement hydraulic chamber 122 when thelockup clutch 32 is disengaged and also discharges the working oil thathas flown out of the second pressure adjusting valve 116 when the lockupclutch 32 is engaged, a bypass port 140 that discharges the working oillocated in the disengagement hydraulic chamber 126 when the lockupclutch 32 is engaged, a relief valve 142 into which the working oil issupplied that has flown out of the second pressure adjusting valve 116,a spring 144 that biases the spool valve element 130 towards the OFFposition, and a hydraulic chamber 146 that receives a switching signalpressure P_(SW) from the switching solenoid valve SL on the end surfaceof the spool valve element 130.

The lockup control valve 106 is provided with a spool valve element 148,a spring 150 that provides a thrust directing the spool valve element148 to a slip (SLIP) position, a hydraulic chamber 152 that receives ahydraulic pressure P_(ON) acting inside the engagement hydraulic chamber122 of the torque converter 30 to bias the spool valve element 148towards the slip position, a hydraulic chamber 154 that receives ahydraulic pressure P_(OFF) acting inside the disengagement hydraulicchamber 126 of the torque converter 30 to bias the spool valve element148 toward the complete engagement (ON) position, a hydraulic chamber156 having supplied thereto a signal pressure P_(SLU) outputted from thelinear solenoid valve SLU for slip control, an input port 158 havingsupplied thereto a second line pressure P_(L2) adjusted by the secondpressure adjusting valve 116, and a control port 160 having suppliedthereto a hydraulic pressure outputted from the bypass port 140 of theclutch switching valve 104. FIG. 4 shows a state in which the spoolvalve element 148 is positioned so that the section on the left sidefrom a center line is in the slip (SLIP) position and shows a state inwhich the spool valve element 148 is positioned so that the section onthe right side from a center line is in the complete engagement (ON)position.

The linear solenoid valve SLU for slip control serves to control thedifference pressure ΔP, that is, an engagement pressure (engagementhydraulic pressure) that actuates the engagement of the lockup clutch 32when the lockup clutch 32 is engaged (slip-engaged). The linear solenoidvalve SLU for slip control outputs the signal pressure P_(SLU) inresponse to a command from the electronic control unit 80. Thus, it isan electromagnetic control valve that takes a constant modulatorpressure P_(M) adjusted by the third pressure adjusting valve 118 as abase pressure and reduces the modulator pressure P_(M) to output thesignal pressure P_(SLU). This valve generates the signal pressureP_(SLU) proportional to a drive current (excitation current)corresponding to a command supplied from the electronic control unit 80.Further, a drain port 162 of the linear solenoid valve SLU for slipcontrol communicates with a check ball 164 and is therefore constantlyclosed by the check ball 164. Where a pressure equal to or higher than apredetermined pressure is applied to the check ball 164, the drain portis opened and the working oil is discharged.

The switching solenoid valve SL outputs a predetermined switching signalpressure P_(SW) in response to a command from the electronic controlunit 80. Thus, in the de-energized state (OFF state), the switchingsignal pressure P_(SW) is taken as a drain pressure, and in theenergized state (ON state), the switching signal pressure P_(SW) iscaused to act as a modulator pressure P_(M) on the hydraulic chamber146, whereby the spool valve element 130 of the clutch switching valve104 is moved to an ON position (ON) which represents the engaged state.

With the hydraulic control circuit 100 configured in the above-describedmanner, the supply state of the working hydraulic pressure to theengagement hydraulic chamber 122 and the disengagement hydraulic chamber126 is switched, and the operation state of the lockup clutch 32 isswitched. First, the case in which the lockup clutch 32 is set to a slipstate or lockup ON state will be explained. Where the switching signalpressure P_(SW) is supplied to the hydraulic chamber 146 by theswitching solenoid valve SL and the spool valve element 130 is biased tothe ON position in the clutch switching valve 104, the second linepressure P_(L2) supplied to the input port 136 is supplied from theengagement port 134 to the engagement hydraulic chamber 122 via theengagement oil path 120. The second line pressure P_(L2) supplied to theengagement hydraulic chamber 122 becomes a hydraulic pressure P_(ON). Atthe same time, the disengagement hydraulic chamber 126 communicates withthe control port 160 of the lockup control valve 106 from thedisengagement port 132 via the disengagement oil path 124 and throughthe bypass port 140. The hydraulic pressure P_(OFF) inside thedisengagement hydraulic chamber 126 is then adjusted by the lockupcontrol valve 106 (that is, the difference pressure ΔP, that is, theengagement hydraulic pressure, is adjusted by the lockup control valve106), and the operation state of the lockup clutch 32 is switched withinthe range from the slip state to the lockup ON state.

More specifically, when the spool valve element 130 of the clutchswitching valve 104 is biased to the engagement (ON) position, that is,when the lockup clutch 32 is switched to the engagement state, thesignal pressure P_(SLU) for biasing the spool valve element 148 to thecompete engagement (ON) position in the lockup control valve 106 is notsupplied to the hydraulic chamber 156, but where the spool valve element148 is set to the slip (SLIP) position by the thrust of the spring 150,the second line pressure P_(L2) supplied to the input port 158 issupplied from the control port 160 via the bypass port 140 and from thedisengagement port 132 via the disengagement oil path 124 to thedisengagement hydraulic chamber 126. The flow rate of the working oiloutputted from the control port 160 is controlled by the signal pressureP_(SLU) supplied to the hydraulic chamber 156. Thus, in a state in whichthe spool valve element 148 is set to the slip (SLIP) position, thedifference pressure ΔP is controlled by the signal pressure P_(SLU) ofthe linear solenoid valve SLU for slip control, and the slip state ofthe lockup clutch 32 is controlled.

Further, where the signal pressure P_(SLU) for biasing the spool valveelement 148 to the complete engagement (ON) position in the lockupcontrol valve 106 is supplied to the hydraulic chamber 156 when thespool valve element 130 of the clutch switching valve 104 is biased tothe ON position, the second line pressure P_(L2) is not supplied fromthe input port 158 to the disengagement hydraulic chamber 126, and theworking oil from the disengagement hydraulic chamber 126 is dischargedfrom the drain port. As a result, the difference pressure ΔP assumes amaximum value and the lockup clutch 32 assumes a completely engagedstate. In a slip state or a completely engaged state of the lockupclutch 32, the clutch switching valve 104 is set to the ON position.Therefore, the relief port 142 is caused to communicate with thedischarge port 138. As a result, the working oil that has flown out ofthe second pressure adjusting valve 116 is supplied to the oil cooler108 via the discharge port 138.

Meanwhile, where the switching signal pressure P_(SW) is not supplied tothe hydraulic chamber 146 and the spool valve element 130 is set to theOFF position by the biasing force of the spring 144 in the clutchswitching valve 104, the second line pressure P_(L2) supplied to theinput port 136 is supplied from the disengagement port 132 to thedisengagement hydraulic chamber 126 via the disengagement oil path 124.Further, the working oil that has been discharged via the engagementhydraulic chamber 122 to the engagement port 134 via the engagement oilpath 120 is supplied from the discharge port 138 to the oil cooler 108and cooled. Thus, in a state in which the spool valve element 130 of theclutch switch valve 104 is positioned in the OFF position, the lockupclutch 32 assumes the disengagement state, and no slip engagementcontrol is performed via the linear solenoid valve SLU for slip controland lockup control valve 106. In other words, even when the signalpressure P_(SLU) outputted from the linear solenoid valve SLU for slipcontrol has been changed, as long as the spool valve element 130 of theclutch switch valve 104 is positioned in the OFF position, the change inthe signal pressure is not reflected in the engagement state (differencepressure ΔP) of the lockup clutch 32. Thus, the clutch switch valve 104can block the supply of the engagement hydraulic pressure to the lockupclutch 32 regardless of the operation state of the linear solenoid valveSLU for slip control. In other words, it is possible to prevent thedifference pressure ΔP serving as the engagement hydraulic pressure fromassuming a positive value.

FIG. 5 is a functional block diagram illustrating the main features ofcontrol functions provided to the electronic control unit 80. As shownin FIG. 5, the electronic control unit 80 is provided with a prechargecontrol unit, that is, a precharge control device 170, and a flexiblestart control unit, that is, a flexible start control device 172. Theprecharge control device 170 shown in FIG. 5 performs a prechargecontrol for establishing a standby state in which a predeterminedpreparation pressure is indicated to the linear solenoid valve SLU forslip control, before the flexible start control by the below-describedflexible start control device 172 is started. Thus, the prechargecontrol device 170 determines whether the predetermined prechargecontrol start conditions that has been set in advance have beenfulfilled, and performs the precharge control when the precharge controlstart conditions have been fulfilled. More specifically, in theprecharge control, a hydraulic control is performed by which apredetermined preparation pressure is indicated to the linear solenoidvalve SLU for slip control, and the actual output pressure P_(SLU)thereof is gradually increased. More specifically, as shown in thebelow-described FIG. 6, an output command that changes in a pulse-likemode is issued to the linear solenoid valve SLU for slip control, and apredetermined (constant) drive current corresponding to thepredetermined preparation pressure is supplied to the linear solenoidvalve SLU for slip control. When the below-described flexible startcontrol start condition is fulfilled, the flexible start control isstarted and therefore the precharge control device 170 ends theprecharge control. The preparation pressure as referred to hereinaboveis an indication pressure that maintains a state in which the spoolvalve element 148 of the lockup control valve 106 is located in the slipposition. Thus, the preparation pressure is empirically set as anindication pressure that is as high as possible, but generates only athrust that is lower than the trust of the spring 150 holding the spoolvalve element 148 of the lockup control valve 106 in the slip position.

Further, when the precharge control is performed, the precharge controldevice 170 blocks the supply of the engagement hydraulic pressure to thelockup clutch 32 with the clutch switching valve 104. Thus, a state inwhich the spool valve element 130 of the clutch switching valve 104 islocated in the OFF position is maintained and the difference pressure ΔPis prevented from assuming a positive value, while maintaining the OFFstate of the switching signal pressure P_(SW) from the switchingsolenoid valve SL.

The specific precharge control start conditions are as follows: (a) thebrake switch 56 detects that a step-on operation of the foot brake pedal54, which is the normal brake, that is, the vehicle brake operation thatbrakes the wheels is canceled (brakes are OFF), (b) the vehicle speedsensor 66 detects that the vehicle speed V is 0, that is, that thevehicle is stopped, and (c) the lever position sensor 74 detects thatthe operation position P_(SH); of the shift lever 72 is a drive range (Drange). In other words, the precharge control start conditions arefulfilled when all of the conditions (a) to (c) are fulfilled. From thestandpoint of these precharge control start conditions, in order tocause the vehicle 8 to move, the operation position P_(SH) is set to theD range as the vehicle is in a stopped state in which the wheels of thevehicle 8 are braked by the wheel brake device 42. Therefore, theprecharge control device 170 starts the precharge control as the brakingof the wheels is canceled when the vehicle starts moving.

A large number of other conditions may be added as the precharge controlstart conditions to the above-described conditions (a) to (c). Forexample, in addition to the fulfillment of the above-describedconditions (a) to (c), the following conditions may be required to befulfilled as the precharge control start conditions: (d) the AT oiltemperature T_(OIL) detected by the AT oil temperature sensor 78 isequal to or higher than a predetermined temperature, (e) the revolutionspeed N_(E) of the engine 28 detected by the engine revolution speedsensor 58 is equal to or higher than a predetermined speed, (f) othercontrol for example such as speed change control by the automatictransmission 10 is detected not to be performed, (g) no failure isdetermined to be caused by the hydraulic control circuit 100 or thelike, and (h) an engagement prohibition requirement of the lockup clutch32 is determined to be absent with respect to other control. Thus, theprecharge control start conditions may be considered to be fulfilledwhen all of the conditions (a) to (h) are fulfilled.

The flexible start control device 172 performs the flexible startcontrol of slip-engaging the lockup clutch 32 when the vehicle startsmoving. Thus, when a predetermined flexible start control startcondition that has been set in advance is fulfilled, the lockup clutch32 is set to a slip engagement state and part of the output torque ofthe engine 28 is inputted to the automatic transmission 10 via thelockup clutch 32, whereby vehicle moving control is performed by whichpower for moving the vehicle is transmitted via the fluid in the torqueconverter 30 and the lockup clutch 32. The flexible start control startcondition is fulfilled, for example, when an operation amount Acc of theaccelerator pedal 50 detected by the accelerator operation amount sensor52 is equal to or greater than a predetermined value (accelerator is ON)and the vehicle speed V detected by the vehicle speed sensor 66 is equalto or greater than a predetermined value, that is, when the wheels aredetected to start rolling. When the flexible start control startcondition is fulfilled, the flexible start control start device 172determines to start the flexible start control.

More specifically, when the flexible start control is started, theflexible start control device 172 switches the clutch switching valve104 to a state in which the engagement hydraulic pressure can besupplied to the lockup clutch 32. Thus, the spool valve element 130 ofthe clutch switching valve 104 is moved from the OFF position to the ONposition by setting the switching solenoid valve SL from thenon-energized state to the energized state. The difference pressure ΔPacting upon the lockup clutch 32 via the linear solenoid valve SLU forslip control can thus be adjusted. The flexible start control device 172performs the flexible start control via the linear solenoid valve SLUfor slip control. Thus, the output pressure P_(SLU) from the linearsolenoid valve SLU for slip control is feedforward controlled bycontrolling a command (drive current) supplied from the electroniccontrol unit 80 to the linear solenoid valve SLU for slip control. Withsuch a control, the difference pressure ΔP is controlled so that theactual slip revolution speed N_(S)(=N_(E)−N_(IN)) of the lockup clutch32 becomes a preset target slip value N_(SM). By performing suchflexible start control, the increase in the revolution speed of theengine 28 when the vehicle starts moving is inhibited and good fuelconsumption can be obtained when the vehicle starts moving. In thepresent embodiment, the fuel consumption is the traveling distance perunit amount of consumed fuel and the improvement of fuel consumption isthe increase in the traveling distance per unit amount of consumed fuelor the decrease in the fuel consumption rate (=(amount of consumedfuel)/(drive wheel output)) of the entire vehicle. Conversely, thedegradation of fuel consumption is the decrease in the travelingdistance per unit amount of consumed fuel or the increase in the fuelconsumption rate of the entire vehicle.

Further, it is preferred that the flexible start control device 172indicate a predetermined surge pressure (surge output value) to thesolenoid valve SLU for slip control immediately after the flexible startcontrol has been started. The surge pressure is designed to supply adrive current of a predetermined value for a predetermined time intervalto the solenoid valve SLU for slip control to accelerate the rise of thedifference pressure ΔP (=P_(ON)−P_(OFF)) of the lockup clutch 32,thereby issuing an output command that changes in a pulse-like manner,such as shown by a broken line L01 in the below-described FIG. 6, to thesolenoid valve SLU for slip control.

FIG. 6 is a time chart illustrating an example of precharge controlperformed by the precharge control device 170. The time chartrepresenting conventional technology in FIG. 6 shows an indication valueto the linear solenoid valve linear solenoid valve SLU for slip controlin response to the indication value in the case in which the flexiblestart control is started without performing the precharge control, as inthe conventional method. In FIG. 6, the indication value (indicationpressure) to the linear solenoid valve SLU for slip control isrepresented by a dash-dot line, and the output pressure P_(SLU), whichis an actual pressure, is represented by a solid line. In order tocompare the control of the present embodiment with the conventionalcontrol, the output pressure P_(SLU) in the conventional control isshown by a thick broken line superimposed on the time chart of thecontrol of the present embodiment.

In the example shown in FIG. 6, at a point in time t1, braking of thewheels is detected to be canceled by a signal from the brake switch 56.Therefore, in the control of the present embodiment, at the point intime t1, the precharge control start conditions, including otherconditions, are fulfilled and the precharge control by the prechargecontrol device 170 is started. Thus, the fast-fill is performed withrespect to the linear solenoid valve SLU for slip control, while theswitching signal pressure P_(SW) from the switching solenoid valve SL isOFF, and then a predetermined drive current corresponding to theindication of the preparation pressure is applied and gradual increasein the output pressure P_(SLU) is started. At this stage which precedesa point in time t2, the switching signal pressure P_(SW) from theswitching solenoid valve SL is OFF and the spool valve element 130 ofthe clutch switching valve 104 remains located in the OFF position.Therefore, the output pressure P_(SLU) from the linear solenoid valveSLU for slip control produces no effect on the difference pressure ΔP ofthe lockup clutch 32 and the output pressure P_(SLU) increases, but theengine revolution speed N_(E) and the turbine revolution speed N_(T) donot change. In the control of the present embodiment illustrated by FIG.6, the output pressure P_(SLU) matches the preparation pressure betweenthe points in time t1 and t2.

By contrast, in the conventional control, the precharge control is notperformed and therefore the output pressure P_(SLU) from the linearsolenoid valve SLU for slip control is not increased and is the samebefore and after the point in time t1.

Then, at the point in time t2, the accelerator depression amount Accbecomes equal to or greater than the predetermine value (accelerator ON)and the flexible start control start condition is fulfilled. Therefore,the precharge control is ended and at the same time the flexible startcontrol by the flexible start control device 172 is started. Thus, thecontrol of the difference pressure ΔP of the lockup clutch 32 by theoutput pressure P_(SLU) of the linear solenoid valve SLU for slipcontrol is started by switching ON the switching signal pressure P_(SW)from the switching solenoid valve SL. This flexible start control is thesame in the conventional control and the control of the presentembodiment, but in the control of the present embodiment, thepredetermined surge pressure (surge output value) may be indicated tothe linear solenoid valve SLU for slip control, as shown by a brokenline L01 in FIG. 6, or the surge output value may not be indicated.

Then, the flexible start control is performed by the flexible startcontrol device 172 before the acceleration flexible control is startedat a point in time t3, that is, within the interval from t2 to t3. Forexample, the flexible start control is ended and the accelerationflexible control is started when the vehicle speed V becomes equal to orhigher than the control end vehicle speed that has been experimentallyset in advance and at which the flexible start control is ended.

The precharge control has been performed at the points in time t1 to t2in the control of the present embodiment (solid line) in order tocompare the solid line with the thick broken line in the time chart ofthe lowermost stage in FIG. 6. As a result, the ability of the outputpressure (actual pressure) P_(SLU) to track the indication value(indication pressure) indicated to the linear solenoid valve SLU forslip control is improved when the flexible start control is started,that is, immediately after the point in time t2. Therefore, in thecontrol of the present embodiment, the output pressure P_(SLU) shiftshigher at the start of the flexible start control than in theconventional control and, as shown in the time shaft of the enginerevolution speed N_(E), the unnecessarily rapid increase in the enginerevolution speed N_(E) as the vehicle starts moving can be inhibitedmore effectively. By so inhibiting the unnecessarily rapid increase inthe engine revolution speed N_(E), it is possible to attain a sufficienteffect in improving fuel consumption.

FIG. 7 is a flowchart illustrating main features of control operationsof the present embodiment performed by the electronic control unit 80,that is, control, operations performed by flexible start control afterthe precharge control has been performed. These operations arecyclically performed with a very short cycle time of, for example, fromseveral milliseconds to several tens of milliseconds.

In FIG. 7, in step (the word “step” is hereinbelow omitted) S1corresponding to the precharge control device 170, it is determinedwhether the precharge control start conditions have been fulfilled. Forexample, where the above-described conditions (a) to (c) are theprecharge control start conditions, it is determined whether all ofthese conditions (a) to (c) have been fulfilled. Where a positivedecision is made in S1, that is, where the precharge control startconditions have been fulfilled, the processing flow moves to S2. Where anegative decision is made in S1, the present flowchart ends.

In S2 corresponding to the precharge control device 170, the prechargecontrol for establishing a standby state in which the predeterminedpreparation pressure is indicated to the linear solenoid valve SLU forslip control is performed. While the precharge control is beingperformed, the switching solenoid valve SL remains in a non-energizedstate. After S2, the processing flow moves to S3.

In S3 corresponding to the flexible start control device 172, it isdetermined whether the flexible start control start conditions arefulfilled. Thus, it is determined whether the operation amount Acc ofthe accelerator pedal 50 that is detected by the accelerator operationamount sensor 52 is equal to or higher than a predetermined value(accelerator ON) and whether the vehicle speed V detected by the vehiclespeed sensor 66 is equal to or higher than a predetermined value, thatis, whether the start of wheel rotation is detected. Where a positivedecision is made in S3, that is, where the flexible start control startconditions have been fulfilled, the processing flow moves to S4. Where anegative decision is made in S3, the processing flow moves to S2 and theprecharge control is continued.

In S4 corresponding to the flexible start control device 172, theswitching solenoid valve SL is switched from the non-energized state tothe energized state and the spool valve element 130 of the clutchswitching valve 104 is moved from the OFF position to the ON position.Then, the flexible start control that slip-engages the lockup clutch 32when the vehicle starts moving is performed. For example, this flexiblestart control is continued till the vehicle speed V reaches the controlend vehicle speed, and when the vehicle speed V becomes equal to orhigher than the control end vehicle speed, the control is switched tothe acceleration flexible control.

Thus, in the present embodiment, the flexible start control device 172performs the flexible start control of slip-engaging the lockup clutch32 when the vehicle starts moving. Further, the precharge control device170 performs the precharge control for establishing a standby state inwhich the predetermined preparation pressure is indicated to the linearsolenoid valve SLU for slip control before the flexible start control isstarted. Therefore, immediately before the flexible start control isstarted, the output pressure P_(SLU) outputted from the linear solenoidvalve SLU for slip control rises to the preparation pressure or ahydraulic pressure close thereto and therefore a sufficient increase inresponse of the lockup clutch 32 in the flexible start control over thatin the case without the precharge control can be ensured. Therefore, forexample, as shown in the time chart in FIG. 6, the unnecessarily rapidincrease in the revolution speed of the engine 28 caused by a delay ofresponse in hydraulic pressure control of the linear solenoid valve SLUfor slip control in the flexible start control can be adequatelyinhibited and fuel consumption can be improved. Further, since theunnecessarily rapid increase in the revolution speed of the engine 28 isinhibited when the vehicle starts moving, the drivability is improved.

Further, with the present embodiment, the precharge control device 170blocks the supply of the engagement hydraulic pressure to the lockupclutch 32 with the clutch switching valve 104 while the prechargecontrol is being performed. As a result, the disengaged state of thelockup clutch 32 can be reliably maintained while the precharge controlis being performed and therefore the lockup clutch 32 can be preventedfrom being actuated to the engagement side before the vehicle startsmoving and the unnecessary decrease in the engine revolution speed N_(E)caused thereby can be avoided.

Further, with the present embodiment, the flexible start control device172 switches the clutch switching valve 104 to the state in which theengagement hydraulic pressure can be supplied to the lockup clutch 32when the flexible start control is started. Therefore, the engagementhydraulic pressure that has been set high in advance by the prechargecontrol can be supplied to the lockup clutch 32 immediately after theflexible start control has been started. Therefore, before the flexiblestart control is started, the disengaged state of the lockup clutch 32can be reliably maintained, and where the flexible start control isstarted, the lockup clutch 32 can be immediately actuated with goodresponsiveness to the engagement side.

Further, with the present embodiment, the precharge control is startedwhen braking of the wheels is canceled. It can be predicted that wherebraking of the wheels is canceled when the vehicle starts moving, forexample, the accelerator pedal 50 will be stepped on and the flexiblestart control will be started immediately thereafter. Therefore, theprecharge control can be started in a suitable period before theflexible start control is started.

The preferred embodiments of the invention are described above indetails with reference to the appended drawings, but the invention isnot limited to these embodiments and other embodiments thereof arepossible.

For example, in the present embodiment, an example is explained in whichthe invention is used to control the lockup clutch 32 in the torqueconverter 30 provided in an FF vehicle, but the invention can be alsoadvantageously used in vehicles of other types, for example, frontengine-rear drive (FR) vehicles.

Further, in the above-described embodiment, the accelerator depressionamount Acc and vehicle speed V are determined as the flexible startcontrol conditions, but it is also possible to determined only theaccelerator depression amount Acc as the flexible start controlcondition, without determining the vehicle speed V.

Further, in the present embodiment, the automatic transmission 10 is amultiple-stage automatic transmission, but it may be also a continuouslyvariable transmission (CVT).

Further, in the present embodiment, the automatic transmission 10 is nota mandatory component, and the vehicle 8 may include no automatictransmission 10. Essentially, it is suffice if the torque converter 30constitute part of the power transmission path between the engine 28 andthe drive wheels 40.

Further, in the present embodiment, the torque converter 30 acts toincrease the transmitted torque as the slip between the input and outputmembers thereof increases, but a hydraulic transmission device thatdemonstrates no such action increasing the transmitted torque may bealso used.

Further, in the present embodiment, the wheels are braked by the wheelbraking device 42 when the foot brake pedal 54 is stepped on, but forexample the electronic control unit 80 may brake the wheelsautomatically with the wheel braking device 42, rather than with thefoot brake pedal 54. When the wheels are thus braked automatically,braking of the wheels may be canceled by an operation other than thecancellation of the step-on operation of the foot brake pedal 54.

While the invention has been described with reference to exampleembodiments thereof, it is to be understood that the invention is notlimited to the described embodiments or constructions. To the contrary,the invention is intended to cover various modifications and equivalentarrangements. In addition, while the various elements of the exampleembodiments are shown in various combinations and configurations, othercombinations and configurations, including more, less or only a singleelement, are also within the scope of the invention.

What is claimed is:
 1. A control device for a vehicular lockup clutch,comprising: a lockup clutch that directly connects an input rotationmember and an output rotation member of a hydraulic transmission deviceconstituting part of a power transmission path between an engine anddrive wheels; a linear solenoid valve that controls an engagementhydraulic pressure for engaging and actuating the lockup clutch; and acontroller that is configured to perform a flexible start control at atime that acceleration is applied for slip-engaging the lockup clutchwhen a vehicle starts moving, and to perform, before starting theflexible start control, a precharge control for establishing a standbystate in which a predetermined preparation pressure is indicated to thelinear solenoid valve.
 2. The control device for a vehicular lockupclutch according to claim 1, further comprising: a switching valve thatblocks a supply of the engagement hydraulic pressure to the lockupclutch regardless of an operation state of the linear solenoid valve,wherein the controller blocks the supply of the engagement hydraulicpressure using the switching valve when the precharge control is beingperformed.
 3. The control device for a vehicular lockup clutch accordingto claim 2, wherein the controller switches the switching valve to astate in which the engagement hydraulic pressure can be supplied whenstarting the flexible start control.
 4. The control device for avehicular lockup clutch according to claim 1, wherein the controllerstarts the precharge control when braking of wheels is canceled.
 5. Thecontrol device for a vehicular lockup clutch according to claim 4,wherein the controller starts the precharge control when braking of thewheels is canceled, the vehicle is stopped, and a shift level is in adrive range.
 6. The control device for a vehicular lockup clutchaccording to claim 1, wherein the controller starts the flexible startcontrol when an operation amount of an accelerator pedal is equal to orgreater than a predetermined value and a speed of the vehicle is equalto or greater than a predetermined value.
 7. The control device for avehicular lockup clutch according to claim 1, wherein a switching signalindicates a switch to the flexible start control.